Torque control systems for conventional port-injected engines are known in which the torque is generally adjusted by controlling ignition timing (spark), fuel quantity or timing, and/or airflow via the throttle plate. Because of the time delay associated with throttle-regulated airflow upstream of the cylinders, and because of the increased pumping losses associated with a throttled airflow, this parameter is generally undesirable or unsuitable for transient torque control. Spark is typically preferred for transient torque control because the engine output torque responds very quickly to changes in spark timing. Furthermore, spark has a broad range of authority over output torque, i.e. spark can be used to rapidly vary output torque to nearly zero, if desired. However, any departure of spark timing from the spark timing that gives maximum torque for a given amount of air and fuel (MBT) may result in a fuel economy penalty.
Transient torque control of conventional engines, whether using spark, air, or fuel, requires consideration of the effect on tailpipe emissions and associated emission control devices. Where three-way catalysts are used, air/fuel ratio must be tightly controlled near the stoichiometric ratio to prevent premature reduction of the catalyst conversion efficiency. Likewise, proper emissions control is more easily achieved by maintaining appropriate catalyst operating temperatures.
Many of the transient torque control strategies used in conventional IC engines have been adapted for use in direct fuel-injected spark-ignition engines, otherwise known as DISI engines. While these strategies may provide acceptable performance for many applications, they do not take advantage of the potential for improved efficiency afforded by differences in operating principles for DISI engines, and in particular lean burn DISI engines. Furthermore, some conventional control strategies are not applicable to DISI engines due to the differences in operation. As such, it would be desirable to provide a system and method for rapid transient torque control of various DISI engine configurations and operating modes which may result in improved engine efficiency and fuel economy.